Rename Result.unwrap_or_handle() to .unwrap_or_else()

Result.unwrap_or_handle() is the equivalent of Option.unwrap_or_else().
In the interests of naming consistency, call it the same thing.

[breaking-change]
auto merge of #14294 : kballard/rust/result_unwrap_or_else, r=alexcrichton

Result.unwrap_or_handle() is the equivalent of Option.unwrap_or_else().
In the interests of naming consistency, call it the same thing.

[breaking-change]